Blow molding has conventionally been used in manufacture of different types of containers and similar articles due to its variety of advantages; it enables production of blow moldings under low pressure to thereby reduce the mold costs, and it improves the strength and rigidity per unit weight due to reduction in weight of moldings. Further, in recent years, blow molding of automobile parts such as bumpers, spoilers, and wheel caps has become of interest. Demands in this field include reduction of molding cycle time for large moldings such as bumpers, and development of a method of manufacturing moldings having excellent appearance such as high surface gloss and the like for spoilers and wheel caps.
In the case in which crystalline resins such as polypropylene resins and polyamide resins are used in manufacture of automobile parts, which require strength and rigidity, even if a mold having a mirror-polished mold inner surface is used, ordinary molding conditions cannot avoid occurrence of melt fractures, die lines, or pockmark-like patterns in resultant blow moldings due to inclusion of gas or crystallization. Thus, in blow molding of a crystalline resin, faithful transfer of mold inner surface can hardly be attained, making difficult the manufacture of blow moldings having excellent appearance in terms of surface gloss and the like.
Japanese Patent Publication (kokoku) No. 2-40498 describes a method for producing blow moldings having excellent surface gloss. According to the method, a parison having a great number of fine projections and depressions (depth: 2-100 .mu.m) in its resin surface is set in molds which are mirror-polished to have a mirror surface of 0.5 S or less and heated to a temperature higher than the crystallization temperature of the resin; blow molding is carried out; and the molds are cooled to a temperature lower than the crystallization temperature. However, this method has disadvantages; surface conditions of a parison must be regulated, resins are limited to those suffering melt fracture upon molding, and molding cycle is long, thereby reducing productivity in manufacture of large moldings such as automobile parts.
In order to improve the surface properties of a blow molding formed of crystalline resin, a conceivable method is one in which a melt parison is used in blow molding and the mold temperature is maintained higher than the crystallization temperature of the crystalline resin when the parison and the molds are in close contact. This method inevitably prolongs the molding cycle. To solve this problem, there have been proposed various methods for effectively cooling the molds heated at a temperature higher than the crystallization temperature to thereby maintain the shape of the molded article. For example, the invention disclosed in Japanese Patent Publication (kokoku) No. 7-77728 is directed to a method in which a high-temperature oil and a low-temperature oil are used as heat media for regulating temperature of molds, and upon switching of the oils, the flow path of the heat media are air-blown. Another invention, which is disclosed in Japanese Patent Application Laid-Open (kokai) No. 6-226828, is directed to a molding method in which molds are heated by use of steam and cooled by use of water. However, these methods in which heating and cooling of molds are performed repeatedly require a plurality of media for heating and cooling, or regulation of temperature of molds over a wide range. Therefore, these methods still have room for improvement in view of the intricate equipment and operation involved. Moreover, repetition of heating and cooling may prevent successful reduction in cycle time of molding. Japanese Patent Application Laid-Open (kokai) No. 4-77231 discloses a blow molding method in which a parison formed of a soft resin containing a crystalline resin is fed into the space between molds, followed by clamping of the molds, and a fluid is pressure-blown into the interior of the parison so as to cause the parison to come into close contact with the mold inner surfaces to thereby carry out molding and cooling, wherein the temperature of the molds is maintained between a temperature in the vicinity of the temperature at which the crystallization rate of the crystalline resin is the highest and the melting point of the crystalline resin, and a fluid serving as a cooling medium is pressure-blown into and circulated within the interior of the parison under pressure. This method may be effective for reducing die lines or weld lines in the surface of a molding; however, for pressure-blowing a cooling medium this method uses two blowing needles of different pressures, and therefore, circulation of the cooling medium occurs only at the initial stage of cooling. Also, since the cooling medium is rapidly heated upon contact with the high-temperature parison, the cooling efficiency is not always satisfactory and thus the reduction in time of molding cycle is limited. Therefore, there remain problems for satisfactory application to production of large blow moldings.
Japanese Patent Application Laid-Open (kokai) No. 6-226829 discloses a method in which a liquid-nitrogen-vaporized gas is fed inside a parison through one of two high-pressure blow pins extending to the interior of the parison and the gas is discharged through the other blow pin, to thereby cool the parison while reducing the molding cycle. However, that patent application discloses no technical concept for attaining both the reduction of molding cycle time and the improvement of mold transferability by regulating the mold inner surface temperature in response to the crystallization temperature of the crystalline resin.